Primary recovery of hydrocarbon (e.g., oil) from a hydrocarbon-bearing (e.g., oil-bearing) reservoirs relies upon the use of natural energy present in the reservoir as the main source of energy for the displacement of oil to production wells. Usually, however, this process merely recovers a minor portion of the original oil in place (OOIP). Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs.
The viability of an oil recovery displacement process depends on two important factors: volumetric sweep efficiency and microscopic displacement efficiency. Enhanced oil recovery (EOR) processes are usually employed to involve the injection of a fluid or fluid of some type into a reservoir. The injected fluids and injection processes supplement the natural energy present in the reservoir to displace oil to a producing well. In addition, the injected fluids interact with the reservoir rock and oil system to create conditions favorable for oil recovery. The mobility control process and chemical process are two commonly used EOR processes.
The widely applied mobility control process is the polymer flood. In a typical application, the polymer solutions are designed to develop a favorable mobility ratio between the injected polymer solution and the oil/water bank being displacement ahead of the polymer. The purpose is to develop a uniform volumetric sweep of the reservoir, both vertically and a really, in order to prevent water from fingering by the oil and moving by the shortest path to the production well. A number of polymer projects have been implemented since 1960's. However, the mobility control process alone does not employ the microscopic displacement efficiency and suffers the low recovery efficiency, thus the incremental oil recovery is limited, usually under 10% OOIP of oil recovery. Manning et al. analyzed statistical data of the fieldwide projects, the median recovery of oil was 2.91% OOIP (1983, Report DOE/ET/10327-19). Schurz et al. summarized results from 99 projects initiated during 1980-1989 and the projected median incremental oil recovery ranges between 3.7% and 4.8% (1989, NMT 890029, New Mexico Tech Centennial Symposium). Gogarty et al discussed about much of incremental recovery by polymer flooding is the result of accelerated oil production before the economic limit is reached (1967, SPE 1566-A, pp. 149-160).
Chemical processes involve the injection of specific liquid chemicals that efficiently displace oil because of the phase behavior properties, which result in decreasing the interfacial tension (IFT) between the displacing liquid and oil. The surfactant/polymer process has been demonstrated to have the potential in application in enhanced oil recovery. In this process, the primary surfactant slug, a micellar solution, is followed by a mobility buffer, a solution that contains polymer which is often graded in concentration, becoming more dilute in polymer as more of the solution is injected. The recovery efficiency primarily uses a displacing fluid that has an ultra low IFT with the displaced oil. Green et al. specifically disclosed that the IFT of displacing fluid must be reduced to ultra low, about 10−3 dyne/cm, before a large reduction in the waterflood residue oil saturation is achieved (1998, ISBN 1-55563-077-4, SPE Textbook Series Vol. 6, pp. 35). There are drawbacks, however. The chemical solutions for generating ultra low IFT, which need to contain surfactant, cosurfactant, and sometimes oil, electrolytes, and alkaline, are usually complicated and expensive, and may suffer chromatographic separation during the EOR operation.
Since the pioneering concept of polymeric soap published by Strauss et al. in 1951, there has been a vast amount of literature published on the polymerization of or in organized amphiphilic assemblies. To some extent, polymeric surfactants serve all the same functions as low molecular weight surfactants. Because of their high molecular weight and complex structures, however, they have some unique characteristics. For example, formation of monomolecular micelles in the dilute solution, various shapes of micelles at different concentrations, etc. Applications such as emulsion stabilizers in submicronic colloidal systems also have been published. Polymeric surfactants are a very attractive class of compounds since the presence of macromolecular chains at the surface of colloidal particles offer significant advantages. This combination of rheological features (e.g. thickening properties) and unique phase behavior properties has broad potential applications in super absorbency, latex paints, hydraulic fluids, flocculation, protein separation, controlled drug release, and biological and medical devices. However, there is only very few literature which explored the use of polymeric surfactant for enhance oil recovery.
The common theory of chemical processes believes that the microscopic displacement efficiency largely determines the residual oil saturation remaining in the reservoir rock at the end of the process, which is one of the key criteria in evaluating the success or failure of a chemical EOR process. Capillary and viscous forces govern phase trapping and mobilization of fluids in porous media and thus microscopic displacement efficiency. Green et al. studied the capillary number Nca=(νμw)/δow, wherein the Nca=capillary number, ν=interstitial velocity, μw=displacing phase viscosity, and δow=the IFT between the displacing and displaced phases (1998, ISBN 1-55563-077-4, SPE Textbook Series Vol. 6, pp. 22). It has been widely accepted in the art that the residue oil saturation cannot be largely reduced unless the δow becomes ultra low at 10−3 dyne/cm level. Therefore, attempts of design polymeric surfactant have so far be concentrated on selecting the polymeric surfactant or preparing the polymer surfactant-containing solution with co-surfactant or other additives to generate low or ultra low IFT value between the oil and water phase.
For example, in early 80s, Chen et al. (1981, U.S. Pat. No. 4,284,517, 1982, U.S. Pat. No. 4,317,893) disclosed a method for the recovery of oil from a subterranean oil reservoir penetrated by spaced injection and production systems in which an aqueous fluid containing polymeric surfactant is introduced into reservoir via injection system to displace oil to said production system. Chen et al. specifically emphasized that the interfacial tension between oil and water should be less than 0.1 dyne/cm (e.g., a preferred the oil-water IFT having a value of 0.005 dyne/cm or less) in order to reach an optimum microscopic displacement efficiency.
Cao et al. (2002, European Polymer Journal, 38 (7), pp. 1457-1463) identified a novel family of polymeric surfactants which might have potential for enhance oil recovery. The novel series of polymeric surfactants is based on carboxy methyl cellulose and alkyl poly (etheroxy) acrylate. The IFT properties of this kind of polymeric surfactant change little with NaCl added. The formed micelles shrink, their size becomes smaller. Alcohols cause the IFT to decrease a little because a small amount of free chains present in solution. Under the influence of added alkali, the IFT of the polymeric surfactants, in aqueous solution, decreases to the level of less than 10−2 dyne/cm.
Influenced by the conventional wisdom of employing ultra low IFT displacing fluid in the chemical processes, even though the hydrophically modified water-soluble copolymers have recently attracted a great deal of interest, the attempt of using polymeric surfactant for the EOR application is mainly aimed at how to generate efficient and stable viscosity to improve the sweep efficiency as mobility controllers. McCormick et al conducted a coordinated, fundamental research program in lab with the ultimate goal of developing “smart” multi-functional polymers that can respond in situ to stimuli and result in significantly improved sweep efficiency in EOR processes (2004, 2005, DOE Report, Award Number DE-FC26-03NT15407). McCormick et al. merely investigated the improvement of sweep microscopic displacement efficiency and phase behavior of polymeric surfactants compared to polymers, but did not disclose the use of polymeric surfactants with oil-water with IFT values more than of 0.1 dyne/cm in EOR.
Contrary to the conventional wisdom, it is unexpectedly discovered that the polymeric surfactants with medium range oil-water IFT value, e.g., no less than about 0.1 dyne/cm (e.g., preferably ranged from about 0.1 to about 15 dyne/cm) have both volumetric sweep efficiency and microscopic displacement efficiency and can be used for hydrocarbon recovery from subterranean formation.